Light emitting diodes (“LEDs”) are well known solid state lighting devices that are capable of generating light. LEDs generally include a plurality of semiconductor layers that may be epitaxially grown on a semiconductor or non-semiconductor substrate such as, for example, sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide substrates. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, some of the electrons will “collide” with a hole and recombine. When this occurs, a photon of light can be emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device (i.e., the area where the electrons and holes recombine).
LEDs typically have a narrow wavelength distribution that is tightly centered about a “peak” wavelength (i.e., the single wavelength where the radiometric emission spectrum of the LED reaches its maximum as detected by a photo-detector). For example, the spectral power distribution of a typical LED may have a full width of, for example, about 10-30 nm, where the width is measured at half the maximum illumination (referred to as the full width half maximum or “FWHM” width). Accordingly, LEDs are often identified by their “peak” wavelength or, alternatively, by their “dominant” wavelength. The dominant wavelength of an LED is the wavelength of monochromatic light that has the same apparent color as the light emitted by the LED as perceived by the human eye. Thus, the dominant wavelength differs from the peak wavelength in that the dominant wavelength takes into account the sensitivity of the human eye to different wavelengths of light.
As most LEDs are almost monochromatic light sources that appear to emit light having a single color, LED lamps that include multiple LEDs that emit light of different colors have been used in order to provide solid state light emitting devices that generate white light. In these devices, the different colors of light emitted by the individual LED chips combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue light emitting LEDs, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the source red, green and blue LEDs.
LEDs are used in a host of applications including, for example, backlighting for liquid crystal displays, indicator lights, automotive headlights, flashlights, specialty lighting applications and even as replacements for conventional incandescent and/or fluorescent lighting in general lighting and illumination applications. In many of these applications, it may be desirable to provide a lighting source that generates light having specific properties.